T5 /'ly 



TS 1109 
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Copy 1 



DEPARTMENT OF COMMERCE 



Technologic Papers 

OF THE 

Bureau of Standards 



S. W. STRATTON, Director 



No. 217 

[Part of Vol. I6J 

PHOTOMICROGRAPHY OF PAPER FIBERS 



R. E. LOFTON, Associate Physicist 

Bureau of Standards 



AUGUST 2, 1922 



1 n ^ '^ 



6^2--( "« 




PRICE, 5 CENTS 

Sold only by the Superintendent of Documents. Government Printing Office, 
Washington. D. C. 



WASHINGTON 

GOVERNIVIENT PRINTING OFFICE 

1922 



^fWWCJPf'' 



LIBRARY OF CONGrttSS 

RECEIVED 

0X4 1922 

DOCUMENTS DIViSfV 






PHOTOMICROGRAPHY OF PAPER FIBERS. 

By R. E. Lofton. 



ABSTRACT. 

This paper describes some of the more important factors in the jihotoiiiicroi^rapliy 
of vegetable fibers, especially of those used in the paper industry. As re^^'ards illumi- 
nation it is shown that the carbon arc can be advantageously replaced by an incan- 
descent stereoptican lamj). DilTcn-nt types of objectives are discussed, and it is 
shown that the working qualities of most objectives may be greatly improved by the 
use of proper light fdters. Three types of light fdters, with general directions for 
preparing them, are discussed, and directions are given for determining the quality 
of light transmitted by filters. The purpose and proper use of the substagc condenser 
and diaphragm are discussed. It is shown that, where an object lying in different 
planes is to be photographed, an objective of comparatively long focal length will 
give better resiilts than one of shorter focal length, and that this arrangement requires 
a longer bellows extension. Other advantages of a long bellows extension are also 
described. Diflerent types of photographic plates are described and suggestions as 
to the best type of plate to use for photomicrographic work are given. Suggestions 
for staining and preparing the material to be photographed are included, as well as 
some suggestions as to the value of photographs for permanent records anrl in the 
study and control of materials and mill processes. A short bibliography on ])h(jto- 
micrography and related subjects is given. 



CONTENTS. 

Page, 

I. Purpose nf publication 629 

II. vSource of illumination 631 

III. Light filters 631 

I\'. I'se (if condensers 636 

V. Use of substage diaphragm 636 

VI. Choice of camera and objective O39 

VII. Photographic plates and their development (144 

YIII. Value of representative field nf fdjers 644 

IX. Value of staining 641; 

X. Photomicrograj)hs as permanent records 647 

I. PURPOSE OF PUBLICATION. 

The purpose of this paper is to put before those who desire to 
take up the microscopic and photomicrograpliic study of the 
vegetable fibers used in paper-making some essentials which are 
often overlooked, but without which the full measure of success 
in this field can not be had. Investigations of the value of stains, 
of light filters, of a long bellows camera as against one of shorter 
bellows, etc., have demonstrated that these factors are really 

620 



-9 



630 



Technologic Papers of the Bureau of Standards. 




COTTOU (Cos 


typiu 


n, iarhaJeme) 


^ ' 
^~^-««" 


rn 


w 




MSP£V (Po^u/us tr».r,:u/o,Vei) 



FLAXCLirum us-itatissimum) 




^i.^.s) 




JACh flNE(Pn,ui dirarica-^a) 




SFRUCB cnouND MOOD 





SKIcer Qvr<lCLifu:clambfr sfyracit'"') 



D0U6LAS SPRUCE f P-eudotsuaa-iaK'- 



Fig. 



-Phnln 



.,ph. of.. 



f-nf^rt fihn 



All vegetable fibers are fairly translucent and have central canals, Cnttnn is n serri hair, ribbon liVc 
olten twisted, bltint ends. Icngrth io-i;=; mm. Manila is a leal-stalk fiber, round, gradually tapering ends, 
length 3-12 mm. Flax and jute are bast fibers, cylindrical, gradually tapering ends. Flax has prominent 
cross markings, often x-shaped. Jute has fine longitudinal lines. Length: Flax, 4-66 mm; jute. 1-5 mm 
Wood fibers come from the trunk and larger branches of trees. Aspen and sweet gum are broadleaf trees; 
fibers are cylindrical, tapering ends. Broad-leaf woods are characterized by comparatively large tubular 
vessels having numerous pores and sometimes bars across either end. Length of broad-leaf fibers: 0.5-2.5 
mm. Jack pine and Douglas spruce are coniferous trees, fibers ribbon-like, blunt or rounded ends, much 
broader than cotton, and have frequent pores. Length coniferous fibers, 1.5-6.0 mm. Ground wood is 
characterized by frequent bundles of fibers showing very distinct cross cells, by broken and mutilated 
fibers, and by extreme v:iriatiun in size of particles. 



i..:fiv„] PJiotomicrotirnphy <'/ Papf) Fibers. 631 

essential in tlie microscopy and especially the jihotoniicroscopy 
of fibrous materials. 

Equally essential, however, is a thorough familiarity with the 
laws and properties of light and lenses, with the proper use of 
the microscope and the fimctions of its various parts, and with 
the nature and properties of develo]3ers and the photographic 
plate. It is beyond the scope of this paper to discuss all these 
subjects, and persons feeling the need of additional information 
will fmd them discussed and treated in detail in the \arious books 
and publications which are availal)le to all.' 

What is said herein regarding libers used in paper-making will 
apply equally well to all iibrous materials, textile as well as ]5aper, 
animal as well as vegetable lil)ers. 

II. SOURCE OF ILLUMINATION. 

In the begiiming of these experiments a small incandescent 
hand-feed arc lamp was used, but the variation in intensity of 
illumination and the frequent adjustment of the feeding mechan- 
ism required by this light, with the consequent periodic diversion 
of the worker's attention, soon led to the attempt to fmd a sub- 
stitute. The use of an incandescent light instead of a carbon 
arc has the additional advantage that a source of heat which has 
often injured or destroyed valuable equipment and delicate 
objects being photographed is eliminated. After some experi- 
menting, a jDo-watt Mazda stereoj^tican lainj) with tulnilar bulb 
was substituted for the carbon arc and found to be satisfactory. 
Photographs at a magnification of 1000 diameters, using a 
Seed 23 plate and a dee]3 blue filter, were made at a n>-minute 
exposure with this lamp. Finally, a new 4()()-watt I\Iazda stereop- 
tican lam]), recently put on the market, was substituted for the 
300-watt lamp. This new lamp is much to be preferred to the 
300-watt, as the six filaments furnishing the light lie in a vertical 
plane, and may thus be turned either edgewise to the object, 
giving an effect similar to that of the Nernst glower, for high 
magnifications, or may be turned flatwise so as to illuminate a 
larger field for low power photomicrography. 

III. LIGHT FILTERS. 

A study of the properties of light and of optical instruments will 
convince the reader, if he has not already been convinced, that all 
optical instruments have their limitations. The best that science 

' .S,i: shiTt bibliusraiihy at end uf ll.is publicitiuii. 




632 TeclDiologic Papers of the Bureau of Standards. [vd. ,t> 

and art can do in the construction of achromatic objectives with 
the different varieties of optical glass available at present is to 
make a perfect correction for two colors of the spectrum, called 
the "preferred" or "chosen" colors. These chosen colors are 
brought to a point focus nearer the objective than are the other 
colors, which are focused at various points along the axis of the 
objective. (See Fig. 2.) The colors chosen for visual work are 

yellow and green, usually 
spoken of as " yellow-green . ' ' 
or " applegreen," because 
the normal human eye is 
most sensitive to this por- 
tion of the spectrum. 
Since the ordinary photo- 
graphic plate is not at all 
sensitive to yellow-green 
light, but is most sensitive 
to the blue-viclet region of tiie spectrum, objectives intended 
e.xclusively for photographic use are corrected for blue light. 
(See Fig. 3.) As photographic objectives of 16 mm equivalent 
focus are comparatively rare, however, almost all photomicro- 
graphic work is done with achromatic outfits constructed for 
visual work, unless, perchance, an apochromatic outfit is at hand. 
It is evident then, since the visual focus and the photographic 
or chemical focus are not the same (see Fig. 4), that best results 
can not be expected if white 
light is used in conjunction 
with an achromatic outfit, 
since white light is composed 
of all the colors of the spec- 
trum. Those who attempt 
to make photomicrographs by 
using an achromatic outht 
and white light are faced at 
once with the problem as to how to make the visual and the chemi- 
cal fdci the same. If a good dry plate, sensitive to yellow-green 
only, were on the market, it would afford the most desirable 
solution of the problem ; for a microscope objective is supposed to 
give the best results, other things being equal, when transmitting 
only those colors of the spectrum for which it is corrected. vSince 
such a photographic plate is not to be had at the present time, 




-Shows objective corrected for 

photographic use, 
loscu (.(ilur" (See text lor explanation) ■ 



Lofion) 



PliotomicicxjKipliy of Paper Filn 



633 




however, the only alternative left is the use of ray or light filters, 
whose function it is to absorb all those rays of the spectrum not 
desirable for illuminating the object to be photographed. 

Light hlters may be purchased, or may be made in the photo- 
graphic workroom which will transmit any portion of tlie spectrum, 
usually within rather broad limits, however. A filter should be 
chosen which will transmit only a very narrow band of the color 
of the spectrum to be used; that is, the light transmitted should 
be as nearly monochromatic as possible. Unfortunately, however, 
hi ters transmitting a very nar- 
row band do not usually trans- 
mit Li large percentage of the 
incident light. 

Three different types of hl- 
ters are in common use: (i) 
Liquid hlters, made liy dis- 
solving various dyes, flepend- 
ing on the color of the light 
wanted, in their proper solv- 
ents; (2) filters made fromcol- 
ored glass ; and (3) filters made 
Ijy staining transparent films 
in xarimis dye solutions. 

Liciuid filters have the dis- 
advantages that they arc usu- 
ally not constant for any 
length of time in the quality 
and quantity of light they 
transmit, that the liquid of 
which they are composed frequently gets spilled or leaks out 
over the apparatus and thus sometimes causes great injury, and 
that they recjuire considerable time and care to keep in ]iroper 
working conditif)n. 

Quite a mmiber of filters made from colored films are on the 
market and some of them are good. There are also available 
glasses of various colors which make excellent filters when ground 
and polished properly. Neither the use of colored films, mounted 
on or between glass, nor that of glass filters has the disadvantages 
attending the use of litiuid filters. 

A suitable filter, often superior to most availal)le, can be made 
by treating an ordinary jihotographic plate in a plain fixing bath 





irocran 






'"' 


■ 


P 


p 


1 


|H 


Ll 


t±u 


.i...,ti- 


^^HH 



The- Ultcrs .It tiK- bolli.ni of i.ich s 


H-cl rocr.in 


to llic culur (if the liitl.t t;illitm on the 


.l.itc; UV 


vioUt; V. vinli-t; B. blue: G, crien 


; V, yell 


..r.aiBi-. .111(1 K. rid The number 




lciii;lh ..I tlR liuht in inillmmh.. ..f .l 


ir.llimet.r 



634 Tcchnoloqic Papers of the Bureau of Standards. iVoi. 16 

in the dark room until all the silver salts are removed, washing, 
and then immersing in an aqueous or aqueous-alcoholic solution 
of a suitable dye for a few hours. 

Although photographic plates may be had sensitive to all colors, 
there are various advantages in using only the shorter wave 
lengths of light, particularly blue light. All plates are more 
sensitive to blue and blue-violet light than to other colors, and so 
when exposed to light of this color require a shorter time of ex- 
posure. (See Fig. 4.) The use of filters transmitting only the 
shorter wave lengths also has the advantage of increasing the 
resolving power of the objective. The theoretical resolving power, 
R, oi a lens — that is, the minimum distance apart of two or more 
dots or lines at which the lens can form a distinct image of each 
dot or line — is given by the formula 

where X is the wave length of the light used and X. A. is the 
numerical aperture of the lens. 

By the use of light filters greater contrast is had between the 
material to be photographed and the background, so that it is 
possible in this way, especially with the use of stains as suggested 
below, to have the printed image stand out in bold relief with no 
background whatever showing. If a blue filter is being used, the 
greatest contrast l)ctween the background and the printed image 
is obtained by staining tlie material to be photographed with red 
or yellow dyes. Black dyes are suitable of course with any light, 
provided the fibers do not take them up in such quantity as to 
prexent all detail of structure from showing. Dense staining 
must be avoided in all cases, sometimes a mere tinting being 
sufficient. 

If one is to get the best results with light filters, there must be 
some practical method for determining the cjuality of light being 
transmitted by the filter used, since this can not be done by the 
unaided eye. The most convenient means of determining the 
transmission of light filters is a replica transmission grating of 
about I by 2 inches in size, having 5000 or 6000 lines per inch. 
To examine the quality of light transmitted by a given filter by 
this means, all light must be excluded from the grating except 
that passing through the filter. The grating must be held between 
the light filter and the eye, and the eye and grating directed some- 
what to one side and not directly toward the light filter. The 



LMfio,,] Photomicrography oj P,ipcr I'ibcrs. 635 

various colors of the light trausniitted can then be seen and the 
quality of the filter determined. 

Another solution of the problem of making the chemical and 
visual foci identical is the employment of an apochromatic objec- 
tive and compensating ocular. If one wishes to use the objective 

without an ocular, however, __ 

a monochromatic filter ' 

should be used, as the apo- - IZZZZZZZZZIZ 

chromat is intentionally so — ZZZHIZZZZZZir 

constructed that, when ZZZZIZIZZZZZ ZZZZZHIIIZIII 

white light is used, it will . — 

not give an image free from — ~~~~~~~~' ' 

color except in combination — ^ 

with acompensatingocular. ' — 

Notwithstanding the finer ~~~~~ZIIIIIIIIZ. ~ ' 

corrections of the apochro- IZIZZll 

mats, it is doubtful if their ZZZZZIZZZZZZ ^_U 

performance in photomi- HHHIZZIZIZZZ ■ 

crography is much superior — ZZIZIZZ ; 

to that of the best achro- ZZZZHZZHZZZ ' 

mats when used Luidcr the ~~~~~IIZIIZZ 

most favorable conditions. 

Besides being less expen- ZZZZZZZZZZIZZ 

sive, the achromats have ~I~ZZZZIIIIII -- 

the advantage of giving a ../L- . ZZZIZ 

flatter image, that is, one ZZZZZIZZZZZZZ ^ __... ...Z 1 .1" 

having less curvature. ' - -.. — . 

Figures 5 and 6, which -— 

are photomicrographs at a , — - j— . — -— j 

magnification of 250 di- 
ameters of a scale ruled on Fig. =;. I-ig. 6. 

a glass slide, show some- .f^'-^f*- Theb(th.alf„fpIintnmirrosrnphsat amag- 

c> ' mhcationof rso-timesof a scale nilcdtin glass, show the rela- 

thing of the relative merits tive flatness ol image given by a 16 mm adimmaticcbiective 

. . .... il'"ig. ^) and b\' a lb mm apochromatic olijective (Fig. M 

of an achromatic objective 

and an apochromatic objective in giving llatness of image, l-'igure 
5, the scale as photographed with the achromatic objective, shows 
that this objective gives a slighter flatter image than the apochro- 
matic objective, with which the same scale was photographed, as 
shown in Figure 6. Both oljjectives have an ecjuivalent focus of 
16 mm. 

lU2.s;i4^— 2l' 2 




tiiiiiiigement uith tu-o converging 
lenses. 



6^6 TccIiHoloqic Papers of ihc Bureau of Sfandards. lVot.,6 

IV. USE OF CONDENSERS. 

For shorter focal lengths than 8 mm a regular substage con- 
denser is not desirable, as better results are to be had by a con- 
denser arrangement which will send a parallel or nearly parallel 
beam of light into the objective. This may be had by removing 
the regular substage condenser and placing at the proper point i:i 

. the beam of light from 

the large condenser 
cither a converging or 
a diverging lens of short 
I'ocal length and rela- 
tively small diameter. 
The converging lens 
must be placed slightly 
outside the point, de- 
pending on its focal 
length, at which the rays are focused by the large condenser (see 
Fig. 7) ; the diverging lens, on the contrary, must be placed slightly 
within this point. (See Fig. 8.) This adjustment is easily carried 
out in a darkened room whose atmosphere contains lloating parti- 
cles of dust or smoke, as the path of the light rays can then be 
distinctly seen. 

V. USE OF SUBSTAGE DIAPHRAGM. 

It is a fact well known to all ])hotographcrs that to close dov.-n 
the diajjliragm of the camera in making a jihotograph increases 
the depth of focus or penetrating jiower of the lens. The only dis- 
advantage the ordinary 
photographer encoim- 
ters in decreasing the 
diameter of the dia- 
phragm is an increase 
in the time of exposure. 
In closing the substage 
diaphragm, however, 
the photomicrographer 

encoimters an additional and far greater evil, namely, the ap- 
pearance of diffraction or false images which mar the defmition 
and perfection of the real image produced by the objective. 
Diffraction eflects are most readily produced in using objectives of 
short focal length, but may be produced with comparatively long 




indcnscr arian:ii:nc:il uiih converging and 
diverging lenses. 



Photomicyography of Paper Fibers. 



L^fic,] Fliotomicrograpny nj raper riueiw. ^37 

focal lengths if the diaphragm opening is made quite small. The 
purpose of the substage diaphragm is to exclude all those rays of 
light, from whatever source, which do not assist in forming the 
most perfect image of which the lens system is capable. Experi- 
ence will soon teach one that the best results are not to be had 
when the diaphragm is wide open, and tliat the less the numerical 




Fio. 



I'lt 



lo. 



iMBS. 



iffects 



Thediaplirasi 
ced lo times to 
iimatic objectiv 



glass, show 
;m openings 



Phot. miiTf graphs at a magnification vi loo times ( 
: to the use ol substage diaphragm of too small apcrt 
were 14 mm. 4 mm. and i mm, respectively. I-'ig. 11 v\as suhsetnicntly ( 
distinctly the effects o( diffraction. All photographs were m.ide with an 
focus and numerical aperture of 0.25. 

aperture of the objective the smaller the diaphragm opening must 
be for the best definition. It has been found that best results are 
obtained with most objectives if the diaphragm is closed down 
until only about three-fourths or three-fifths of the back lens of 
the objective is lighted. The portion of the back lens which is 
lighted can be readily determined, if the mirror is properly ad- 
justed, by removing the ocular and looking down into the tube 



63S Tcchnoloqic Papers of the Bureau of Standards. [Voi.io 

of the microscope. If the diaphragm opening is too large some of 
the rays of light enter at too great an angle with the axis of the 
optical system ; when the diaphragm is much too large a large 
proportion of the rays enter so obliquely that there is produced 
what is called "Hooding" of the objective with light, with the 
result that dellnition is almost or wholly destroyed. This latter 
C(Midition exists, however, only with substage condensers of 
relatively large numerical aperture. 




Fig. iia. — Chemical pulp from aspcn'Kioo. 

Stain . BiMnarck brown 

Mmmiing medium Venetian turpentine 

Objcttive 16 mm achromatic N. A. lo.2$ 

Diaphragm i mm 

Exposure 30 minutes 

Notice the diffraction bands— black and while lines parallel with the outlines of the fiber^^. " Flare 

pots." caused by dust particles being brought partially into focus by the aid of the small diaphragm, are 

evident- Compare with Fiu n and I'ig. 17, 

r'igures 9, 10, 11, and iia illustrate these facts. In Figure 9 
the substage diaphragm was full open, in 10 it was closed to 4 mm 
diameter, and in 11 and iia- it was closed as far as possible, 

* The filter used was made in the laboratory from a photographic plate and a blue dye, by the method 
outlined in this paper. This filter transmits a rather narrow band in the blue and violet, and a small and 
relatively weak band in the deep red. Instead of the regular substage condenser, an achromatic lens of 
5onim focal length was substituted and used throughout the^e experiments. In all photographs Seed No. 
23 plates were used. Metol-hydroquinone developer of about double the usual strength was used for de- 
veloping the plates. The same kind of developer of the usual strength was used in making the prints. 
All prints weremadeon regular glassy velox, and were rolled down onferrotypeplatestodry. The Venetian 
turpentine used as the mounting medium was an artificial product. An attempt was made to secure some 
true Venetian turpentine, but without success. The artificial product, however, gave excellent results, 
although sufl'icicnt time has not elapsed since it was first used here to indicate anything as to its value as a 
permanent mountinij medium. The turpentine as used had a refractive index of 1.520 at 20° C. 



Photomicroqruphy of Paper Fibers. 



639 





to I mm. The sharpness of dehnition of Figure 9 is much better 
than that of 10; the lines in the latter are appreciably broader 
because of diffraction. In Figures 11 and iia diffraction effects 
are more apparent. Figure 11 is a lo-times enlargement of the 
original photomicrograph made at a magnillcation of 100 diameters. 

VI. CHOICE OF CAMERA AND OBJECTIVE. 

Successful ]5hotomicrographs can not be made with a camera 
provided with a very short bellows, especially if a comparatively 
large photograph, an S-inch by lo-inch, 
for example, is wanted at considerable 
magnification. If a short bellows is used, 
an objective of shorter focal length must 
be tised than if the bellows were longer, 
and an objective of short focal length is 
never desirable if one of longer focal 
length will give the detail required in the 
photograph. The objective of long focal 
length has the following advantages over 
one of less focal length: (i) Greater depth 
of focus (2) gives a flatter image, and (3) 
shows a larger field. By depth of focus, 
sometimes called penetrating power, is 
meant the capacity of a lens to bring to 
a sharp focus at the same time an object, 
or objects, lying in slightly separated 
planes perpendicular to the axis of the 
lens. The depth of focus of a lens is di- 
rectly proportional to the scjuare of its 
equivalent focus, or focal length, and in- 
versely proportional to its numerical aper- 
ture. A study of Figures 12, 13, and 14 
will show that the greater the angle 1; which the perijiheral rays 
make it crossing at the focusing point the less will be the 
dejnh of focus. I'^igures 12 and 13 rejiresent lenses of the 
same focal length, but the diameter of 13 is much greater than 
that of 12, and it is very evident that the depth of focus of 

13 is much less than that of 12. Figures 12 and 14 represent 
lenses of the same diameter, but 14 has a much shorter focal 
distance than 12, so that the peripheral rays through 14 make 
a greater angle than through 12; hence the depth of focus of 

14 is less than that of 12. By tlie use of the diaphragm in 



crical aperture, n 
the marginal rays 
:it the principal fo( 



h hnsi-s..|diff.'i 
and different n 
(1= the angle \vl 



640 Technoloqic Papers of the Bureau of Standards. {\\.i. ,6 

photography the peripheral or marginal rays may be "stopped" 
out, thus decreasing the effective diameter of the lens, so that 
objects both relatively near to and relatively distant from the lens 
are in sharp focus. 

Figures 15, 16, 17, and iS illustrate the increasing depth of focus 
of objectives of increasing focal lengths. 

It is shown in treatises on light that all points of real images 
formed by convergent lenses do not lie in the same plane, even 
though the optician may have exhausted his skill in making the 




Fig. 15. — Chemical j^iilf^fipm aspi-n yioo. 

stain _ Bismarck brown 

.Mounting medium Venetian turpentine 

Objective 4 mm acliromatic X. A. /0.85 

Diaphragm 4 nim 

Exposure 4 minutes 

necessary corrections. Although this curvature of image is usually 
not evident in lenses of comparatively long focal length, it is quite 
evident in lenses of such short focal lengths as high power, or even 
moderately high power, microscope objectives. When rays paral- 
lel to the axis are incident at all points on the surface of the lens, 
those rays passing through the margin are brought to a focus at 
points nearer the lens than are those passing through nearer its 
center. Also, of the oblique rays passing through the center of 
the lens those making the greatest angle with the axis are brought 
to the focus nearest the lens. For these reasons the image / of 



Lofion] Photomicrography of Paper Fibers. 



641 




Fig. lb.— Chemical f^ulp fww a.spen Xwo. 

Stain Bismarck brown 

Mounting medium \'enetian turpentine 

Objective S mm achromatic N. A. /0.50 

Diaphragm 4 mm 

Exposure 1 minutes 




Fig. 17. -Chciulcil pulp f)0)n a.^pcn y lOO. 

Stain Hismarck brown 

Mounting medium \'enciian lurpcntine 

Objective 16 mm achromatic X. A. /'o.ss 

Diaphragm - 4 mm 

Kxposure i minutes 



642 



Technologic Papers oj the Bmean oj SiamlarJs. [Voi. 



the object will take the form as shown in Figure 19. Persons 
using the microscope have observed this condition, in tliat to get 
objects lying near the margin of the field of view in focus after the 
center of the field has been sharply focused upon one has to rack 
the objective slightly toward the object. In objectives of longer 




Fig. 18.— Chcmkul pulf>fiom Uipcn yiOO. 

Stain Bismarck brown 

Mounting medium Venetian turpentine 

Objective 48 mm achromatic N. A. /o. 

Diaphragm - - - 4 mm 

Exposure 4 minutes 




t?=nbiect: /= image; 
xplan.ui.m, 



Fig. iq. — Curviiliirc of i»uu:i' of a convex lens. 
-optical center of lens. .1 .! = axis . .1 lens . /■ - principal I. 



focal length this curvature of image does not exist to such an 
extent, so that it is particularly advisable to use objectives of 
longer ecjuivalent focus in photomicrography if a comparatively 
large photograph is to be made. But an objective of longer equiv- 
alent focus will require a longer bellows unless an eyepiece of higher 



U>fUm\ 



PhotoiniciOiji'uphy of Paper I-'ihci; 



^'43 



power be used, which is usually not desirable, since high-power 
eyepieces of ten unduly aggravate faults already existent, or even 
introduce new ones. Although compensating eyepieces must be 
used with apochromatic objectives unless monochromatic light is 
used, the use of eyepieces is not necessary with achromatic objec- 
tives, and not even desirable in most cases, if means are at hand 




IMG. 23. 

Fiss. .'o. n, 22, and -m. photon 
the greater flatness of image give 
achromatic i»bjective, Fij:. ji wit 
jective and Fis -m with a 4^ mm 



21. Fig, 22 1-iG. 2.^ 

t a mufinificatii.nof 100 times of a scale 1 
'es of greater focal leucth Fie ?o was 
chromatic objective. Fig. 22 with a 16 



for getting the desired magnification without their use. If an 
eyepiece must be used, it should be of very low power, not more 
than about two times (2X). All the photographs shown were 
made with achromatic objecli\'es, with but one exception, without 
the use of eyepieces. 

Objectives of long focal length not only give greater depth of 
focus and less curvature of image, but they also give a larger field 



644 Teclniologic Papers of the Bureau of Standarels. [ix. /a 

of view and cover a larger plate than those of shorter focal length, 
so that by their use a larger photograph can be made. It is also 
desirable that the total area covered by the image at the ground 
glass screen be much larger than the plate used to record the 
image, since then only the central portions of the image will be 
photographed and the best focus, as Hat an image as possible under 
the circumstances and the best definition will be secured. Fig- 
ures 20, 21, 22, and 2Ti show that the curvature of the image is 
less with objectives of greater focal length. The glass scale was 
not long enough, however, to show the comparative values of the 
16 mm and 48 mm objectives in this respect. 

VII. PHOTOGRAPHIC PLATES AND THEIR DEVELOPMENT. 

The selection of the dry plate to be used in making a photo- 
micrograph is of almost as much importance as any other factor. 
Various plates were tried during this investigation, and it was 
found that good results were to be had only by using compara- 
tively slow plates. Even with these plates a contrast developer 
was used and was found not to give too great contrast if the stain- 
ing of the fibers was properly done. The slow plate usually has 
greater resolving power, due to the finer grain of the silver salts 
in the gelatine emulsion. These advantages will much more than 
compensate for the additional time of exposure required over that 
of the faster plate. Process plates, however, are not to be recom- 
mended for general work, as they give too much contrast. 
Whether one shoidd use an ordinary, an orthochromatic, or a 
panchromatic plate will depend, of course, on the colors of the 
material to be photographed, the purpose in \iew in making the 
photograph, and perhaps on other factors. 

VIII. VALUE OF A REPRESENTATIVE FIELD OF FIBERS. 

It is of first importance that the microscopist should know the 
distinguishing properties and characteristics of the fibers which he 
is about to photograph and that he should take all precaution to 
get a representative field under the microscope. It would be 
manifestly wrong, for example, to make a photomicrograph of a 
field of aspen soda pulp which does not show any of the character- 
istic pitted vessels chiefly by means of which all hardwoods are 
distinguished from the softwoods or conifers. It also gives a 
wrong impression to see a {photomicrograph of wood pulp showing 
an incorrect proportion of fillers to vessels, tracheids, or paren- 
chyma cells. The material to be photographed, whether it be 



Lofi.;A Pliotoinicicgiiiphy nj Paper Fihei.s, 645 

pulp or other material, should lirst be studied suffieiently under 
the microscope to determine its properties and characteristics, and 
the correct proportion of the various cells or other components, 
by counting or by some other reliable method. A field for photo- 
graphing should then be chosen which will show the correct pro- 
portion of the various individuals m other characteristics which 
it is the pirrpose of the photographer to show. 

IX. VALUE OF STAINING. 

The utility and value of the use of various stains and dyes in the 
differentiation and study of plant tissues and elements has long 
been recognized by botanists. A great number of dyes and stains 
of recognized worth for botanical work are on the market, and this 
number is IxMiig graflually added to. 

A number of these stains are more or less selective in their 
action on jilant tissue, and many of them are emphatically so. 
Their great \-alue lies in the fact that because of this .selective 
property they dye certain tissues or certain parts of fibers more 
deeply than others, and thus bring out or accentuate character- 
istic markings, reactions, or other properties of the fibers. By 
their use the differentiation and identification of various fibrous 
materials, otherwise difficult or impossible, is made cjuite easy and 
certain. 

Their value in the photomicrography of paper-making fibers is 
greater, if possible, than in the visual study of plant fibers. One 
of the most serious limitations to the photographing of fibrous 
materials is that their finer markings and characteristics can not 
be shown as readily and as distinctly loy ])hotographic means as 
by visual means. Hence any means of showing more distinctly 
the characteristic markings of libers is a manifest advantage in 
making a photograph of these fibers; this means is supplied by 
the proper use of stains. 

Another great value oi the use (if a proper stain is the contrast 
afforded between the outlines of the fibers and the backgroimd. 
By this means the fibers may be made to stand out in bold relief, 
with U(j suggestion whatever of a V:)ackgromid in the finished print 
if the work is well done. 

Figures 24 and 25 show the advantages of staining in bringing 
out the physical features of fibers, and especially in offering sufli- 
cient resistance to the passage of the light vays through the libers 
to allow the plate to be cxposerl long enough to prevent the 
background showing in the finished print. 



646 Technologic Papers of the Pjtreau of Standards. iVvii6 



'^. 
•% 



Fig. 24. — Flax fibers Xioo. 



Stain Xone 

Mounting medium . . \'enetian turpentine 

Objective - 16 mm achromatic N. A. ars 

Diapliragiu ' 4 mm 

Exposure 2 minutes 




Fig. 25. — Flaxjihcis Xioo. 

Stain Methylene blue and salTranin 

Mounting medium Venetian turpentine 

Objective. 16 mm achromatic N. A. 0.35 

Diaphragm 4 mm 

Exposure . 3 minutes 



Lrfio,,] Photomicrography oj Paper Fibers. 647 

X. PHOTOGRAPHS AS PERMANENT RECORDS. 

The use to which jjhotomicrograpliy may be jiut in the control 
of various industrial processes and in recording the presence of 
adulterations are innunieralile. " \Vhere\'cr the niicroscojje finds 
application and permanent records of observations are required, 
photomicrography is the readiest, most convenient, and accurate 
means available. I'rints can be stored with the written notes of 
analyses and examinations, and have not only the value of records 
of known facts but of j^oints luiknown or unrecognized at the time, 




Fig. 26.— Fi7'./. fiom all ui.: 1,01 



illlund.' ImiIii 



objective 
I>iaphragin. 



whicli would ine\-ilahly be missed in any written notes, but may 
become of the highest importance in future researches and afford 
accumulated evidence which it would be impossible to obtain 
otherwise." ' 

In the paper industry photomicrography may l)e advantageously 
employed in the study and control of the cooking and beating 
processes, in the e.xamination of pulps and pulp mixtures, and of 
sizing and loading materials, in recording adulterations frequently 
found in dyes, in studying bacteria and molds, dirt in pulp and 



= Hind and Raudk-s, Ha 



Tosraphy, p. ;iS. Limilu 



64S Technologic Papers of the Bureati of Standards. [Voiie 




Fig. 27. — Fihers from oil rag rncdiion -dcight bond Y 100. 

Stain Zinc chloride— Iodine 

Mounting medium Zinc chloride— Iodine 

Objective 16 mm achromatic N. A. 0.25 

r>iaphragm 4 mm 

Exposure lia minutes 




Fig. 28. — Fibers from all la^} thin bond Y loO . 



Stain 

Mounting mcdii 
Objective 

Diaphragm 

Exposure 



Zinc chloride — Iodine 
Zinc chloride— Iodine 
, 16 mm achromatic N. A. 0,25 



i..'/'""l Photomicioqraphy of Puffer Fihos. 649 

paper, and in many other ways. Whatever the eye can see Ijy 
the aid of the microscoj^e may be recorded photogra]ihica]ly if 
proper methods are used. 

Photographs 26, 27, and 28 show the differences in beater treat- 
ment for (26) ledger paper, (27) medium-weight bond, and (28) 
thin bond, and indicate the vahie of photomicrographs as perma- 
nent records. 

BIBLIOGRAPHY. 

A short list of books and articles on photomicrography and 
closely related subjects. 

Light axu Optics. 

r.ausch & Lonib Optical Co., Lenses, tluir liistun-, tlu-dry, Mini manufacture ; Ruches- 

tcr, N. v.. iqoo. 
Heck, Conrail, and .Andrews, Herbert, Photoijraphic lenses, 7th editiun; R. & J. Heck 

(Ltd., I, 6S Cornhill, London. 
Daniell, .\.. .\ text-liook of the iirincii)les of ])hysics: .Macmillan & Co., New York and 

I.flndon. 
Rdser, Edwin, Light for students; Macmillan & Co., London, 1907. ( Hleinentary, 

but very complete, i 
Lummer, O., and Thoni])Son, Silvanus, Contributions to ])hotoj;raphic o])tics; Alac- 

millan & Co., Mew York, kjoo. 
Watson, William, .\ te.xt liook of physics; Lon;<nians, Greene & Co., New York and 

London. 

MiCKoscorv. 

bausch, ]'... jManiind.ction of the microscope: Rochester, N. Y.. iqo6. 

Carpenter-! I, dlini,'er. The microscope and its revelations; I'. lUakiston's Son & Co., 
riuladelphia, loi;, 

Chamot, Rmile Munnin. l{leiTientar>- chemical microscopy; lohn \\'ile\- & Sons, New- 
York, 1020. 

Gage, Simon Henry, The microscope, an introduction to microsci>]iic methods ami to 
histolosy: The Comstock I'ublishins,' Co.. Ithaca, N. Y., lo.-o. (Yen," g I.l 

Hanausek, T. h'., and Winton, Andrew L , The microscopy of technical products; 
John Wiley S: Sons, New York, i.,07. 

Hogg, Jabe/:, The microscope, its histor)-, construction, and a|i]ilication; Geon;e 
Routledge & Sons (Ltd.), London, igii. (Very complete.) 

Lee, A. B., The microscopist's vade-mecum; P. Blakiston's Son & Co., Philadelphia, 

Microscopy of pulpwoods. Paper 26, 1!); April ji, 1020. 

vSpitta, Edmund, Microscopy, the construction, theon,-, and use f>f the microscope; 
John Murn,-, London, iqc(). 

Stokes, Alfred, Aquatic microscopy for beginners, John Wiley & Sons, New York, 
19x8. 

The microscope, its design, construction, and applications, J. Roy. Microscopical Soc, 
Part 4; Dec, 11120, .See copies of above in Engineer, 120, p. 85-87, Jan. 23, iq2o, 
and in Engineering, 109, p. 86-87, ^1"^ 106/7, Jan. 16 and 2:;, 1Q20. (See current 
and b.ack numbers of J. Roy. Microscopical Soc. and of Tran. of Am. Microscopical 
Soc. for articles dealing with all phases of microscopical work. 1 

WTiipple, G. C, The microscopy of drinking water; John Wiley S: S<jns, New York, 
1914. 



LIBRARY OF CONGRESS 

llilfllSllillliiii^: 

018 478 381 2 • 

650 Technologic Papers of flic Bureau of Standards. [Voi. 16 

I'HoTllGRAPHY. 

Bavlev, R. Child, The complete ])hotographer; McClure, Phillips & Co., New York, 
1907. 

Derr, Louis, Photography for students of physics and chemistry; Macmillan & Co., 
New York, iqo6. 

Flint, William Ruthven, Chemistry for photographers; American Photographic Pub- 
lishing Co., Boston, igi6. (See current and back numbers of the various photo- 
graphic periodicals, American and British, and especially the British Journal 
Photographic Almanac, published annually by Henn.- Greenwood & Co., 24 Welling- 
ton Street, Strand, London. ) 

Johnson, George L., Photography in colours; E. P. Button & Co., New York, 1017. 

Taylor, J. Traill, The optics of photography and photographic lenses; Whittaker & Co., 
New York, 1904. 

Tennant, John A., Coloring lantern slides; Tennant & Ward. New York, 1907. (Write 
the publishers, 10^ Park Avenue, New York, for a list of publications dealin^; with 
all phases of photography. ) 

Photomicrography. 

Barnard, J. Edwin, Practical photomicrography; E. Arnold, I.X)ndon, iqii. 

Central scientific company (460 East Ohio St., Chicago, 111.), Bulletin 95S, "gives a 

complete explanation of the application of color filters to photomicrography. 
Experiments in ray collecting and filtering for microscopists; Sci. Am. Sup. S6, 407; 

June 29, 1918. 
Gage, S. H. and H. P., Optic projection; Comstock Publishing Co., Ithaca, N. Y., 

1914. (See especially chapters 9, "The projection microscope," 10, "Drawing and 

photographv with projection apparatus," and 11, "Optics of projection.") 
Hind, H. Lloyd, and Randies, W. Brough. Handbook of photomicrography; George 

Routledge & Sons ( Ltd. ). I^ndon, 191 v ( One of the ver\- best 1 jooks in English on 

this subject. ) 
Light filters and histokigical dyes for ]i!iiitomicrographic work; Brit. J. Photography, 

a(j. p. iQi. 

Washington, January 31, ig22. 

J* 



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